Hybrid indirect-drive/direct-drive target for inertial confinement fusion

ABSTRACT

A hybrid indirect-drive/direct drive for inertial confinement fusion utilizing laser beams from a first direction and laser beams from a second direction including a central fusion fuel component; a first portion of a shell surrounding said central fusion fuel component, said first portion of a shell having a first thickness; a second portion of a shell surrounding said fusion fuel component, said second portion of a shell having a second thickness that is greater than said thickness of said first portion of a shell; and a hohlraum containing at least a portion of said fusion fuel component and at least a portion of said first portion of a shell; wherein said hohlraum is in a position relative to said first laser beam and to receive said first laser beam and produce X-rays that are directed to said first portion of a shell and said fusion fuel component; and wherein said fusion fuel component and said second portion of a shell are in a position relative to said second laser beam such that said second portion of a shell and said fusion fuel component receive said second laser beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Division of application Ser. No. 14/261,991filed Apr. 25, 2014, which claims benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Patent Application No. 61/826,598 filed May 23, 2013entitled “Hybrid Indirect-Drive/Direct-Drive Target for InertialConfinement Fusion,” the disclosure of which is hereby incorporated byreference in its entirety for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

BACKGROUND Field of Endeavor

The present invention relates to relates to inertial confinement fusion,inertial fusion energy, and more particularly to a hybridindirect-drive/direct-drive target for inertial confinement fusion.

State of Technology

In inertial confinement fusion (ICF), a driver—i.e., a laser, heavy-ionbeam or a pulse power system—delivers an intense energy pulse to atarget containing around a milligram of deuterium-tritium (DT) fusionfuel in the form of a hollow shell. The fuel shell is rapidly compressedto high densities and temperatures sufficient for thermonuclear fusionto commence. The goal of present ICF research is to obtain ignition andfusion energy gain from a DT target. The gain of an ICF target isdefined as the ratio of the fusion energy produced to the driver energyincident on the target and is a key parameter in determining economicviability of future inertial fusion energy power plants. The two primarymethods of driving ICF targets are “indirect-drive” and “direct-drive.”

The National Ignition Facility (NIF) is presently seeking to demonstratelaser-driven ICF ignition and fusion energy gain in the laboratory forthe first time by means of indirect-drive. In the latter, the laserenergy is first converted to x-rays in a hohlraum surrounding the fuelcapsule and the x-rays then perform the ablatively-driven compression ofthe capsule. Direct-drive is an alternative method of imploding ICFtargets where the laser beams impinge directly on the capsule surfaceand directly cause ablation compression. In both cases, ignition isinitiated by the PdV work of the high-velocity converging shellstagnating on a central hotspot. Applicant can define this ignitionmethod as “fast compression ignition”.

SUMMARY

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides a hybrid indirect-drive/direct driveapparatus for inertial confinement fusion utilizing laser beams from afirst direction and laser beams from a second direction, comprising: acentral fusion fuel component; a first portion of a shell surroundingsaid central fusion fuel component, said first portion of a shell havinga first thickness; a second portion of a shell surrounding said fusionfuel component, said second portion of a shell having a second thicknessthat is greater than said thickness of said first portion of a shell;and a hohlraum containing at least a portion of said fusion fuelcomponent and at least a portion of said first portion of a shell;wherein said hohlraum is in a position relative to said first laser beamand to receive said first laser beam and produce X-rays that aredirected to said first portion of a shell and said fusion fuelcomponent; and wherein said fusion fuel component and said secondportion of a shell are in a position relative to said second laser beamsuch that said second portion of a shell and said fusion fuel componentreceive said second laser beam. The present invention includes thehybrid indirect-drive/direct drive apparatus for inertial confinementfusion further comprising a fill tube extending through said firstportion of a shell or said second portion of a shell to said fusion fuelcomponent.

The present invention provides a hybrid indirect-drive/direct drivemethod for inertial confinement fusion utilizing laser beams from afirst direction and laser beams from a second direction, comprising thesteps of: providing a unit of fusion fuel, assembling a first portion ofa shell having a first thickness partially surrounding the fusion fuelunit, assembling a second portion of a shell having a second thicknessgreater than the first thickness of the first portion of a shellpartially surrounding the fusion fuel unit to complete the shell,assembling a hohlraum containing at least a portion of the fusion fuelunit and at least a portion of the first portion of a shell in aposition relative to the first laser beam, shock igniting the firstportion of a shell and the fusion fuel using the first laser beam toproduce X-rays that are directed to the first portion of a shell and thefusion fuel; and shock igniting the second portion of a shell and thefusion fuel using the second laser beam. The present invention includesa hybrid indirect-drive/direct drive method for inertial confinementfusion further comprising the step of using a fill tube extendingthrough the shell to inject fusion fuel into the unit of fusion fuel.

The present invention provides a hybrid, high-gain target for inertialconfinement fusion that combines the symmetry advantages ofindirect-drive fuel assembly with the efficiency of radial-direct-driveshock ignition in a capsule with thick fuel shells. A slow, thickspherical shell segment of fusion fuel is assembled on a high-densitymetal guide cone (e.g. gold) by indirect radiation drive in a one-sidedhohlraum. It is then shock ignited on the opposite side byradial-direct-drive on a corresponding spherical fuel segment inside thecone. The two fuel segments communicate hydrodynamic energy and momentumat late time via a hole at the cone tip. Such a target is well suitedfor the laser beam geometry of the National Ignition Facility becausethe direct-drive side is pure radial; thus it would not require a futurepolar-direct-drive qualification campaign or new phaseplates in thefinal optics and will minimize laser cross beam transfer. Its naturaltwo-sided laser illumination geometry and high-gain prospects also makeit attractive for future inertial fusion energy power plants.

The present invention has use as a high gain target for inertial fusionenergy power plants. The present invention has use as an inertialconfinement fusion platform for the National Ignition Facility to obtainthermonuclear ignition and fusion energy gain.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIG. 1A illustrates an embodiment of a hybrid target, FIG. 1Billustrates a Prior Art Direct Drive target, and FIG. 1C illustrates aPrior Art Indirect Drive target.

FIG. 2 provides a comparison of the features and critical issues for:(1) conventional indirect-drive, (2) polar-direct-drive shock ignitionand (3) this new two-sided hybrid platform, as they relate to fieldingon the National Ignition Facility.

FIG. 3 delineates the main differences in the configuration andoperation of the hybrid target relative to impact fast ignition.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings, to the following detailed description, and toincorporated materials, detailed information about the invention isprovided including the description of specific embodiments. The detaileddescription serves to explain the principles of the invention. Theinvention is susceptible to modifications and alternative forms. Theinvention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides a hybrid, high-gain target for inertialconfinement fusion that combines the symmetry advantages ofindirect-drive fuel assembly with the efficiency of radial-direct-driveshock ignition in a capsule with thick fuel shells. A slow, thickspherical shell segment of fusion fuel is assembled on a high-densitymetal guide cone (e.g. gold) by indirect radiation drive in a one-sidedhohlraum. It is then shock ignited on the opposite side byradial-direct-drive on a corresponding spherical fuel segment inside thecone. The two fuel segments communicate hydrodynamic energy and momentumat late time via a hole at the cone tip. Such a target is well suitedfor the laser beam geometry of the National Ignition Facility becausethe direct-drive side is pure radial; thus it would not require a futurepolar-direct-drive qualification campaign or new phaseplates in thefinal optics and will minimize laser cross beam transfer. Its naturaltwo-sided laser illumination geometry and high-gain prospects also makeit attractive for future inertial fusion energy power plants.

Definition of “direct drive:” In the “direct drive” method energy isdelivered to the outer layer of the target using high-energy beams. Theheated outer layer explodes outward, producing a reaction force againstthe remainder of the target, accelerating it inwards, compressing thetarget. A Prior Art Direct Drive target 100 b with laser beams 104 bconverging on a pellet 120 b is illustrated in FIG. 1B.

Definition of “Indirect drive:” In the “indirect drive” method thelasers heat the inner walls of a gold cavity called a hohlraumcontaining the pellet, creating a superhot plasma which radiates auniform “bath” of soft X-rays. The X-rays rapidly heat the outer surfaceof the fuel pellet, causing a high-speed ablation, or “blowoff,” of thesurface material and imploding the fuel capsule in the same way as if ithad been hit with the lasers directly. Symmetrically compressing thecapsule with radiation forms a central “hot spot” where fusion processesset in—the plasma ignites and the compressed fuel burns before it candisassemble. A Prior Art Indirect Drive target 100 c with laser beams104 b converging on a hohlraum 106 c and a pellet 102 c is illustratedin FIG. 1B.

The National Ignition Facility (NIF) is presently seeking to demonstratelaser-driven ICF ignition and fusion energy gain in the laboratory forthe first time by means of indirect-drive. In the latter, the laserenergy is first converted to x-rays in a hohlraum surrounding the fuelcapsule and the x-rays then perform the ablatively-driven compression ofthe capsule. Direct-drive is an alternative method of imploding ICFtargets where the laser beams impinge directly on the capsule surfaceand directly cause ablation compression. In both cases, ignition isinitiated by the PdV work of the high-velocity converging shellstagnating on a central hotspot. Applicant can define this ignitionmethod as “fast compression ignition”.

The attractive features of indirect-drive include the radiationsmoothing of low-mode laser drive asymmetries by the hohlraum and strongablative stabilization of Rayleigh-Taylor instabilities due to thedeeply penetrating x-rays. It is, however, inefficient due to the lowconversion efficiency of laser energy to x-rays in the hohlraum. Notonly does this result in only modest fusion energy gain but necessitatesthin, high aspect ratio, high velocity fuel shells in order to achieveignition; such thin shells are susceptible to breakup and mix that mayimpair the attainment of the ignition temperature.

By contrast, because the laser impinges directly on the fuel capsule,direct-drive is a more efficient at converting laser energy intohydrodynamic motion of the shell and higher fusion energy gains canresult. However, it lacks the smoothing features of indirect radiationdrive and thus the imploding shell can be more susceptible to asymmetryand stability issues. Moreover, the laser beams on NIF are configuredfor indirect-drive—that is, they are arranged in fourhemispherical-opposed cones from 23.5 deg to 50 deg in order to threadthrough holes at the top and bottom ends of the hohlraum—whereas, inprinciple, direct-drive requires symmetric drive beams over 4€ solidangle. Tests of direct-drive on NIF are possible in “polar-direct-drive”where the beams are retained in the present up-down, indirect-drive portconfigurations but where sufficient drive uniformity may be achievableby a combination of beam repointing and partial defocusing. The latterfix may also incur cross-beam power transfer where incoming beams fromone direction scatter power off beams refracted from other directions.Finally, while fully spherical beam illumination geometry is ideallyrequired for direct-drive target performance, it is not an optimumgeometry for an inertial fusion energy power plant because of the largenumber of penetrations required through the target chamber vessel forthe beam ports. A two-sided-drive geometry is much preferred.

In the case of direct-drive (but not indirect-drive), in addition to theconventional ignition method of fast compression of a high velocityshell on the hotspot, a developing ignition concept called “shockignition” is under study where compressed fusion fuel is separatelyignited by a strong late time shock. Here, the fuel assembly andignition phases are decoupled as follows: The cryogenic shell isinitially imploded on a low adiabat using a laser main drive of modestpeak power and lower total energy. While the resulting low implosionvelocity yields only a low temperature central region, the low adiabatof the fuel leads to high values of the assembled areal and massdensities. The compressed fuel is then separately ignited from a centralhotspot heated by a strong, spherically-convergent shock driven by ahigh intensity spike at the end of the laser pulse. The launching of theignition shock is timed to reach the center just as the main fuel isstagnating and starting to rebound. Because the implosion velocity issignificantly less than that required for conventional(fast-compression) hotspot ignition, considerably more fuel mass can beassembled for the same kinetic energy in the shell. This larger burningfuel mass then provides higher fusion gains/yields for the same laserdrive energy or, equivalently, retaining acceptable gains at lower driveenergies. Shock ignition can be considered a more efficient way toperform direct-drive and higher target gains result. However, beingdirect-drive, it still ideally requires the uniformity of full symmetricbeam illumination and thus on NIF will have to depend onpolar-direct-drive illumination with possibly attendant cross beamenergy transfer issues.

It should be noted that that it is not feasible to achieve shockignition in pure indirect-drive. While the drive laser would be capableof providing the required fast rise of the shock pulse, the resultingradiation drive temperature rises only slowly due to the thermal inertiaof the hohlraum. Secondly, because the shock is not launched until latetime where the capsule has converged to around one third of its originalradius, the now large case-to-capsule ratio results in low radiationcoupling efficiency. The result is that only a weak decaying shock wouldbe launched through the shell with no contribution to the centraltemperature at stagnation.

An alternative and more speculative approach to inertial confinementfusion presently under study is “fast ignition” that, like shockignition, seeks to decouple fuel assembly from the ignition process, andmay circumvent some of the above issues that encumber both conventionalindirect and direct-drive. However, fast ignition requires twophysically distinct, time-synchronized laser systems—a main “slow” laserdriver (˜20-40 ns) to compress the fuel and a separate fast,ten-petawatt-class laser (˜10's ps), to create a high energy (˜MeV)electron beam in the target to ignite the fuel. In particular, given thevery demanding timing and spatial focusing requirements, present studiessuggest that fast ignition may not be viable without some breakthroughin the efficiency of the energy channeling from the fast igniter laserbeam to the ignition hotspot. Because of these issues, there are noplans to attempt fast ignition on NIF in the foreseeable future.

A variant of fast ignition known as “impact fast ignition” attempts tocircumvent the difficulty of channeling the energy of the high power,short pulse laser into the high energy electron beam by instead causingthe fast laser to drive a thin, high velocity (2×10̂8 cm/s) flyer platethat stagnates against the compressed fuel. This approach has thecritical issues of inflight breakup of the flyer plate during toinstabilities and mix of the flyer plate material with the high densityfuel during stagnation that can prevent the hotpot from igniting. Thisapproach shares with the hybrid target the concept of employing a guidecone for spherical fuel segments. The major difference is that impactignition is a fast ignition variant in that the fuel is assembledisochorically (constant density) with no low density hotspot and is thenfast ignited by the impacting flyer plate. By contrast, the hybridtarget employs conventional isobaric assembly such that the high densitycold compressed fuel and low temperature hotspot are in pressureequilibrium during stagnation and ignition of the hotspot is produced byshock ignition. Other design and operational differences are highlightedin FIG. 3 below

The present invention provides a new concept for a hybrid, high-gainignition target for inertial confinement fusion that combines thesymmetry features of indirect-drive fuel assembly with the efficiency ofradial-direct-drive shock ignition in a capsule with thick fuel layers.

The present invention provides a hybrid, high-gain target for inertialconfinement fusion that combines the symmetry advantages ofindirect-drive fuel assembly with the efficiency of radial-direct-driveshock ignition in a capsule with thick fuel shells. A slow, thickspherical shell segment of fusion fuel is assembled on a high-densitymetal guide cone (e.g. gold) by indirect radiation drive in a one-sidedhohlraum. It is then shock ignited on the opposite side byradial-direct-drive on a corresponding spherical fuel segment inside thecone. The two fuel segments communicate hydrodynamic energy and momentumat late time via a hole at the cone tip. Such a target is well suitedfor the laser beam geometry of the National Ignition Facility becausethe direct-drive side is pure radial; thus it would not require a futurepolar-direct-drive qualification campaign or new phaseplates in thefinal optics and will minimize laser cross beam transfer. Its naturaltwo-sided laser illumination geometry and high-gain prospects also makeit attractive for future inertial fusion energy power plants.

Conventional indirect-drive offers low mode drive symmetry and strongablative stabilization during the capsule drive. It is, hover,inefficient, requires thin, high velocity fuel shells and results in lowfusion energy gains. By contrast, shock ignition in direct-drive offershigher drive efficiency in slow, thick shells but implementation ideallyrequires fully symmetric laser beam illumination which is not optimumfor inertial fusion energy plant applications; testing of shock ignitionon NIF will necessitate polar-direct-drive illumination and potentiallynew phaseplates in the laser final optics and may incur the penalty ofcross beam energy transfer.

In this hybrid concept, Applicant exploits the advantages of bothapproaches in a hybrid target configuration. A design example is shownin FIG. 1A. A slow, thick ˜250 deg spherical shell segment of DT fuel isassembled on a high density metal guide cone (e.g., gold) byindirect-drive in a one-sided hohlraum. The gold surface is coated witha low-atomic number anti-mix layer, e.g. C. It is then shock ignited onthe opposite side via direct-drive on a ˜110 deg spherical fuel segmentinside the cone. Given that the target has a natural two-sided symmetryand the direct-drive side is pure radial drive over a ˜110 deg fuelsegment, it doesn't require a polar-direct-drive qualification campaignon NIF or new phaseplates and should eliminate cross beam for transferbetween the direct-drive beams. Its two-sided illumination geometry andhigh-gain prospects also make it attractive for future inertial fusionenergy plants.

The two fuel segments both comprise thick layers of solid cryogenic DT.On the indirect-drive side, the fuel shell segment is backed by anablator shell segment comprising plastic (CH) or other low-atomic-numbermaterial such as Be, diamond, SiC, B4C, etc., containing a smallfraction (˜2-4% atomic) of higher atomic number dopant (e.g., Si) toabsorb the high-frequency M-band radiation from the hohlraum. The radialcomposition of this shell is similar to those under consideration forthe present NIF indirect-drive target for the National Ignition Campaign(hereinafter the “NIC” target), except the DT fuel layer is much thickerand the ablator is thinner (see design specifications below). Thedirect-drive side segment is simply a very thick layer of solidcryogenic DT that acts as both ablator and fuel; approximately half theDT ablates outwards during the drive and the other half is compressedinwards. A thin (˜10-15 ìm) plastic CH seal coat is required on theoutside of this all-DT segment but this burns off early in the laserdrive. If it should prove difficult to produce smooth solid DT “ice”layers on each side of the cone using the conventional “beta-layering”technique established for the NIC target, Applicant has the option ofemploying liquid DT wicked into low density (˜25 mg/cc) CH foam shells.This latter variant adds the possible complication of impurity mix inthe DT fuel, but because the DT fuel is in liquid form it would notexhibit the surface structure (“roughness”) of solid frozen DT that canform a seed source for instability growth.

The two converging fuel segments communicate energy and momentum at latetime via a ˜50-100 μm diameter hole at the cone tip. The hotspot on thedirect-drive side is shock ignited by a spherically-converging shockdriven the high intensity spike at the end of the direct-drive laserpulse with the hotspot tamped by the assembled areal density of thestagnated fuel on the indirect-drive side. The ignition energy from thedirect-drive shock ignition side is transmitted to the indirect-driveside and a thermonuclear burn wave propagates into the cold compressedfuel of the latter. Given that the majority (˜80%) of the DT fuelresides on the indirect-drive side, a corresponding majority of thetotal thermonuclear yield accrues from that side.

FIG. 1A illustrates the general layout of the indirect-dive/direct drivetarget for inertial confinement fusion of the present invention. Thefollowing numbered components are illustrated in FIG. 1A:

-   -   100 HYBRID INDIRECT-DRIVE/DIRECT-DRIVE TARGET ASSEMBLY    -   102 GOLD HOHLRAUM    -   104 GOLD HOHLRAUM AI BACKUP    -   106 GUIDE CONE    -   108 INDIRECT-DRIVE FUEL SEGMENT    -   110 DIRECT-DRIVE FUEL SEGMENT    -   112 CENTER LINE    -   114 INDIRECT-DRIVE LASER BEAM ARRAY    -   116 DIRECT-DRIVE LASER BEAM ARRAY    -   118 GAS FILLED VOLUME D-T GAS    -   120 INNER LAYER SOLID DEUTERIUM-TRITIUM (D-T)    -   122 OUTER LAYER CH PLASTIC+2% Si

The reference number 100 indicates the target assembly that issymmetrical about the centerline 112. The main components that comprisethe target assembly 100 are the gold hohlraum 102 and it's aluminumbacking 104, a guide cone 106 an indirect-drive fuel segment 108 and adirect-drive fuel segment 110. The two polar laser beams arrays theindirect-beam array 114 and the direct-drive beam array 116. Some ofthese items will be described in greater detail in FIG. 2.

In FIG. 1A Applicant shows the design specifications for a high gainexperimental test platform for this hybrid target for fielding on NIF.The DT fuel layer thicknesses on the indirect-drive is 210 μm and inthis design example uses a Si-doped (2%) CH plastic ablator of thickness120 μm; alternative ablator materials could include Be, diamond, SiC,B4C, etc. The direct-drive side comprises a 350 μm-thick segment of DTthat acts as both fuel and ablator within a thin 15 μm seal coat. Thetwo fuel layers are factors of ×3 and 5× thicker than the fuel layer forthe present NIC baseline design, an important consideration indetermining stability and corresponding potential for shell breakup andmix.

Applicant performed two-dimensional (2D) design optimizations with theLASNEX radiation-hydrodynamics implosion code on the prospective NIPhybrid platform shown in FIG. 1A. Applicant first optimizes theimplosion dynamics of the indirect-drive segment to maximize the arealdensity of the stagnated fuel layer by tuning the laser drive pulseshape in the hohlraum. This stagnated fuel shell then acts as a tampmass for the ignition hotspot initiated on the other side. Theindirect-drive assembly side requires a one-sided drive energy of 0.71MJ at a peak power of 157 TW, resulting in a hohlraum peak radiationtemperature of 248 eV. (This should be compared with the present NICtarget that requires a two-sided drive of ˜1.8 MJ at 500 TW and ˜300eV). Applicant then optimizes the foot and main drive of the laser pulseshape on the direct-drive ignition side to maximize the areal density ofthat stagnating shell segment, followed by tuning of the launch time ofthe ignition shock pulse to maximize the temperature in the ignitionhotspot. The radial direct-drive shock ignition side requires aone-sided drive of ˜0.53 MJ at 231 TW (This should be compared withprospective polar-direct-drive shock ignition targets for NIF thatrequire two-sided (polar) drives of ˜0.7-1 MJ at ˜350-450 TW). Finally,because the radial-direct-drive is more efficient that theindirect-drive compression, synchronizing the two segments to optimizethe 2D burn dynamics—that is, maximize the fusion yield and energygain—requires that the start time of the former drive pulse must bedelayed by ˜8.2 ns after the start of the indirect-drive side.

Under the above burn optimization, this target achieves a 2D fusionyield of 39 MJ and thus a gain of ˜32. Applicant also observed a uniqueburn history for this platform in that the rate of production of fusionenergy as a function of time exhibits a double maxima as the smaller,faster direct-drive side ignites and the burn wave then propagates intothe main fuel mass on the other side. With conventional single shelltargets—indirect or direct-drive, fast compression ignition or shockignition—only a single narrow maximum is observed in simulations.

The primary critical issues for this target that may ultimatelydetermine ignition and fusion yield performance include (1) Stability ofthe two fuel segments during inflight convergence and late timestagnation (Such stability considerations attend all classes of inertialconfinement fusion targets). (2) The achievement of adequate implosionsymmetry in a one-sided hohlraum during the assembly phase of theindirect-drive side (Although Applicant notes that, unlike conventionalindirect-drive, Applicant is not seeking high velocity nor ignitionhere, rather just the attainment of a reasonable stagnation arealdensity to tamp the hotspot formed on the other side) (3) Minimizinghigh-atomic-number mix into the hotspot from the gold guide cone thatseparates the two fuel segments. A low-z (e.g., C) anti-mix surfacecoating is applied to the cone to ameliorate this. (Recent simulationsof an analogous guide cone for a fast ignition target indicates that thedrag on the sliding DT plasma at the Au/DT interface tends to leave theAu mix behind). Future simulations and proof of principle experimentswill determine the importance of these issues.

FIG. 2 illustrates a fill tube used with the indirect-dive/direct drivetarget for inertial confinement fusion of the present invention. Thefollowing numbered components are illustrated in FIG. 2:

-   -   200 Half section of target assembly 100    -   102 Gold hohlraum 20 um thick    -   104 Hohlraum aluminum backing    -   108 Indirect-drive fuel segment    -   202 Outer layer CH plastic+2% Si    -   204 Inner layer solid deuterium-tritium (D-T) 50:50 atomic        fraction 210 um thick    -   206 Gas filled volume D-T gas 0.0003 glcm3    -   208 Outer layer CH plastic (no Si) 15 um thick    -   210 Inner layer solid deuterium-tritium (D-T) 50:50 atomic        fraction 350 μm thick    -   106 Gold support cone    -   212 Fill tube

FIG. 2 is a half section 200 of the target assembly 100. Theindirect-drive fuel segment 108 is made up of two layers, an outer layer202 of CH plastic+2% Si and an inner layer 204 of solidDeuterium-tritium (D-T) 50:50 atomic fraction 204 um thick. Thedirect-drive fuel segment 110 has two layers, the outer layer 208 is CHplastic (no Si) and the inner layer 210 is solid Deuterium-tritium (D-T)50:50 atomic fraction and is 350 um thick. The hollow volume 206 of thetwo fuel segments 108 and 110 is filled with D-T gas 0.003 glcm3. Asshown the two fuel segments 108 and 110 are mounted on the cone section106 of the hohlraum.

FIG. 3 illustrates an indirect-drive/direct drive target assembly withalternate fuel layers. The following numbered components are illustratedin FIG. 3:

-   -   300 Hybrid indirect-drive/direct drive target assembly with        alternate fuel layers    -   102 Gold hohlraum    -   104 Gold hohlraum aluminum backup    -   106 Guide cone    -   108 Indirect-drive fuel segment    -   110 direct-drive fuel segment    -   112 Center line    -   202 Outer layer CH plastic+2% Si    -   304 Inner layer low density (0.025 glcm3) CH foam (aerogel)        filled with liquid D-T    -   206 Inner gas volume    -   208 Outer layer CH plastic (no Si)    -   310 Inner layer low density (0.025 glcm3) foam (aerogel) filled        with liquid D-T

Although the description above contains many details and specifics,these should not be construed as limiting the scope of the invention butas merely providing illustrations of some of the presently preferredembodiments of this invention. Other implementations, enhancements andvariations can be made based on what is described and illustrated inthis patent document. The features of the embodiments described hereinmay be combined in all possible combinations of methods, apparatus,modules, systems, and computer program products. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination. Similarly, whileoperations are depicted in the drawings in a particular order, thisshould not be understood as requiring that such operations be performedin the particular order shown or in sequential order, or that allillustrated operations be performed, to achieve desirable results.Moreover, the separation of various system components in the embodimentsdescribed above should not be understood as requiring such separation inall embodiments.

Therefore, it will be appreciated that the scope of the presentinvention fully encompasses other embodiments which may become obviousto those skilled in the art. In the claims, reference to an element inthe singular is not intended to mean “one and only one” unlessexplicitly so stated, but rather “one or more.” All structural andfunctional equivalents to the elements of the above-described preferredembodiment that are known to those of ordinary skill in the art areexpressly incorporated herein by reference and are intended to beencompassed by the present claims. Moreover, it is not necessary for adevice to address each and every problem sought to be solved by thepresent invention, for it to be encompassed by the present claims.Furthermore, no element or component in the present disclosure isintended to be dedicated to the public regardless of whether the elementor component is explicitly recited in the claims. No claim elementherein is to be construed under the provisions of 35 U.S.C. 112, sixthparagraph, unless the element is expressly recited using the phrase“means for.”

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

The invention claimed is:
 1. A hybrid indirect-drive/direct drive methodfor inertial confinement fusion utilizing laser beams from a firstdirection and laser beams from a second direction, comprising the stepsof: providing a unit of fusion fuel, assembling a first portion of ashell having a first thickness partially surrounding said fusion fuelunit, assembling a second portion of a shell having a second thicknessgreater than said first thickness of said first portion of a shellpartially surrounding said fusion fuel unit to complete said shell,assembling a hohlraum containing at least a portion of said fusion fuelunit and at least a portion of said first portion of a shell in aposition relative to said first laser beam, shock igniting said firstportion of a shell and said fusion fuel using said first laser beam toproduce X-rays that are directed to said first portion of a shell andsaid fusion fuel; and shock igniting said second portion of a shell andsaid fusion fuel using said second laser beam.
 2. The hybridindirect-drive/direct drive method for inertial confinement fusion ofclaim 1 further comprising the step of using a fill tube extendingthrough said shell to inject fusion fuel into said unit of fusion fuel.